The present invention relates to an image recording apparatus for color-correcting image signals output from an image signal source and outputting image signals to record an image on an image recording medium.
There is known an image recording apparatus, as disclosed in Japanese Patent Laid-Open Publication No. Hei. 3-69369, which processes image signals from various types of color image signal sources, such as computer graphics systems, and outputs visible image signals onto an image recording medium. To record an image with a good color reproduction on the image recording medium using the image recording apparatus, color correction must be made in consideration of the color reproduction characteristics of the image signals from the image signal source, the image recording apparatus and the image recording medium. Further, the setting of the color correction must be frequently adjusted for, for example, the color adjustment according to the user's taste, and the compensation for the nonuniformity of the image record media.
It has been known to employ the combination of two look-up tables and two exponential converters provided for each of three colors (FIG. 4). According to the image output unit, a first look-up table converts image signals from an image signal source into a region substantially linear with respect to optical density. As a result of the linear conversion in the density region, the colors can be adjusted substantially along the visual characteristics of a human being. With provision of the combination of a second look-up table and an exponential converter in the succeeding stage, good gray balance can be ensured independently of the nonuniformity of the recording media. The same can also be maintained when the optical density and hardness are adjusted.
Generally, complicated nonlinear relationships are present between input image color signals (R.sub.i, B.sub.i, and G.sub.i respectively representing red, green, and blue) and coloring materials (C.sub.0, M.sub.0, and Y.sub.0 respectively representing cyan, magenta, and yellow) of the recording medium: EQU C.sub.0 =f(R.sub.i, G.sub.i, B.sub.i) EQU M.sub.0 =g(R.sub.i, G.sub.i, B.sub.i) EQU Y.sub.0 =r(R.sub.i, G.sub.i, B.sub.i) (1)
The color reproduction ranges of the input image color signals are frequently different from those of the recording medium. Particularly in a case where the input image signals are displayed on a color CRT monitor, when an image is recorded on a reflecting medium it is necessary to realize the compression of a preferable image reproduction region using formula (1) since the color reproduction range of the CRT monitor is very wide.
Also for color adjustment, it is necessary not only to consider human visual characteristics but also to use a color adjusting device designed in consideration of the input image signal characteristics.
Use of only the look-up tables provided for the respective colors allows the apparatus to effect only the correction for the relations given by formula (2). Therefore, the resultant color reproduction is unsatisfactory. EQU C.sub.0 =f(R.sub.i) EQU M.sub.0 =g(G.sub.i) EQU Y.sub.0 =r(B.sub.i) (2)
Further, the adjustment of hue and saturation is also impossible. The color balance adjustment, for example, is based on the hue of the coloring materials of the recording medium. Accordingly, the results of the adjustment do not always result in a natural-looking image.
A three-dimensional look-up table has been known for accurately correcting the relations of the formula (1). The look-up table is addressed with the respective colors of the input image signals. Where the input image color signal and the output image color signal are each expressed by 8 bits, a table memory of approximately 50M bytes (2.sup.8.times.8.times.8 .times.3.apprxeq.50M) is required. Use of such a large memory is uneconomical. Additionally, for the color adjustment, the table of 50M bytes must be rewritten, leading to poor efficiency of the color adjusting work.
In some recent usage of the system, a single image output unit is connected to a plurality of host computer graphic systems or the image output unit is located at a remote place or used by a plurality of users. When considering this trend of usage, it would be desirable to make the color adjustment from the host system. However, in the color adjustment, it is difficult to well manage the standard conditions of the image output unit.
When the image data transfer speed, the color correction processing speed of the image output unit, and the image recording speed are not matched, much time is taken for the image recording operation or, to avoid this, a large frame memory is required for the image output unit.
Accordingly, with view of solving the problems as stated above, the present invention has an object to an image recording apparatus which has a good color reproduction property and makes an easy color adjustment.